Most people have the impression that they read by gliding their gaze across the page. However, in fact, it has been known since around 1900 that we read by taking 0.2 second static glimpses (fixations) separated by quick jerks (saccades). The brain integrates the information from the glimpses over time to create the illusion of taking it all in at once, as we glide across the page.
The visual span, or the uncrowded window, is the number of characters that the observer can take in without moving his eyes. This limits how much the observer can perceive during each fixation. Letters beyond the visual span are too crowded to be recognized. Each successive glimpse advances to the right by about one visual span. Reading speed is equal to the product of saccade rate and saccade size. The saccade rate is about four per second for most readers across a wide range of conditions. The saccade size is approximately the visual span. This suggests that increasing the visual span would increase reading speed, by the same proportion. However, conjectured methods for increasing the visual span depend on assumed the theory for what limits the visual span.
An object that is surrounded by other objects (clutter) may be hard to identify. One sees only a jumble. This is crowding. Acuity sets a minimum letter size. Crowding sets a minimum letter spacing. Like acuity, crowding gets worse with increasing eccentricity. The maverick theory is that visual span is limited by crowding (i.e. required letter spacing) not acuity (i.e. required letter size). The required letter spacing is negligible at fixation and grows in proportion to distance from fixation. For uniformly spaced text, the letters near fixation will be far enough apart to be read. Letters too far from fixation will be spaced less than the critical spacing apart and will not be readable. The uncrowded window, centered on fixation, is the visual span. Reducing crowding may increase the visual span and speed up reading.
Most attempts to alleviate crowding rely on the fact that crowding depends on similarity. Increasing the difference between objects (in this case, letters) reduces crowding and increases saliency. For example, in a pair of letters, both black, on a blank field, the letters crowd each other, but in a different pair, one black and one white, the letters do not crowd each other, because of the color difference. However, in a line of many letters, alternately black and white, adjacent characters do crowd each other. This is because of grouping. Crowing is the result of grouping. In a pair of characters, the visual system groups letters of the same color together, so they crowd. An isolated unlike pair does not group and does not crowd. In the long line of alternating characters, the regular alternation of color is perceived as a pattern (stripes) that groups, so there is crowding. The trick to reduce grouping and crowding is to increase the differences without creating patterns. This also results in increasing saliency, which increases attention. Reduced crowding and increasing attention can increase the visual span and speed up reading.